You may want her to use an enema first in addition to a lubricant like astroglide or wet (I believe some of these have special applicators for that specific use) in addition to a shower to gether before andafter for cleaning, relaxing and foreplay.
No risk for any STD's but just make sure you DON'T insert your penis into her vagina after anal unless you clean yourself really good. If you do she can get all kinds of infections there and it won't be pretty.
In order for you to get any sort of secxual transmitted disease your partner would have to have one. So if you both are indeed clean then you can not get an std.